Pigment flakes formed of one or more thin film layers have been developed for a wide variety of applications. For example, magnetic pigments are used in products ranging from security devices to decorative cookware. Similarly, color shifting pigments are used in cosmetics, anti-counterfeiting inks, automobile paints, and so on. Various thin-film flakes and methods of their manufacturing are disclosed e.g. in U.S. Pat. Nos. 5,571,624, 4,838,648, 7,258,915, 6,838,166, 6,586,098, 6,815,065, 6,376,018, 7,550,197, 4,705,356 incorporated herein by reference.
The pigment flakes are conventionally manufactured using a layered thin film structure formed on a flexible web, also referred to as a deposition substrate. The various layers are deposited on the web by methods well known in the art of forming thin coating structures, such as Physical and Chemical vapor deposition and the like. The thin film structure is then removed from the web material and broken into thin film flakes, e.g. color shifting flakes, which can be added to a polymeric medium such as various pigment vehicles for use as ink, paint, or lacquer.
In the process of removing the deposited layer(s) from the deposition substrate, the coating breaks into flakes, which may further be reduced in size, e.g. by milling. The random breakage of the coating results in irregularly-shaped and irregularly-sized flakes as shown in FIG. 1. One of disadvantages of this approach is the presence of flakes which are too small for a particular application. By way of example, flakes having a diameter of less than 3 microns diminish optical effects of the final product coated with the pigment flakes. Sieves are conventionally used in order to remove flakes of undesired size, or debris. However, using sieves adds complexity to the flakes manufacturing process and results in wasting a portion of deposited material. Accordingly, it is desirable to increase utilization of coating materials, decrease waste, and lessen complexity of flakes manufacturing process.
Furthermore, in some applications, the shape of pigment flakes is important. By way of example, flakes of a particular shape may be used as taggants, or markers, in security or anti-counterfeiting applications as taught in U.S. Pat. No. 7,258,915.
It is known that planar non-magnetized magnetic flakes align in a magnetic field with their long axes parallel to flux lines of the magnetic field. When magnetic flakes have a diffraction grating thereon, it is highly desirable to coordinate the directions of the grating and of the long axis and, thus, to have control over the flake's shape.
U.S. Patent Appl. No. 20090072185 describes a higher reflectivity coating formed of square-shaped flakes assembled into ribbons.
In a conventional method of manufacturing shaped flakes, a deposition substrate is embossed with frames so as to induce breaking of the coating and to obtain shaped flakes. The resulting flake mixture includes leftover frames separated from the flakes. By way of example, FIG. 2 shows 8×8 microns pigment flakes with a large number of broken frames which have been separated from the flakes during post-processing. Thus, this method produces a significant amount of undesirable debris in the form of flake frames.
An object of the present invention is to provide a method of manufacturing shaped thin-film flakes which produces practically no debris.